Vehicles having snow plow blades affixed to a front end thereof are in common use. Most of such plow blades are releasably mounted on the truck. As is well known, such plow blades have a substantial mass and are very heavy. Moreover, the plow blades are mounted to the frame of the vehicle for movement between a lowered working or operational position and a raised storage or suspended position.
Typical plow suspension systems include a mounting structure or frame attached to the forward end of the vehicle. It is common for such mounting structures to be releasably attached to the frame of the vehicle. Usually, the plow blade is hung or suspended forwardly of the mounting structure in a manner permitting vertical movement. A lift assembly operably positions the plow blade between its suspended transport or raised position and its operating or lowered position. Such lift assembly usually includes a lift arm adapted for movement about a pivotal axis. A driver controls movement of the lift arm and thereby the plow blade. A chain or cable system typically interconnects the plow blade to the lift arm.
As the vehicle is driven across uneven terrain or surfaces, i.e. railroad crossings, ruts, potholes and the like, the elevated plow blade can present significant problems and major difficulties. More specifically, when the plow blade is not in its lowered or work engaging position, the momentum imparted to the plow blade as the vehicle is driven over uneven terrain causes the suspended plow blade to bounce. That is, the uneven surface terrain causes initial upward movement of the plow blade toward the lift arm and subsequent forceful movement downwardly until the chains or cable limit its travel. Without any plow suspension system, when the plow blade reaches the travel limit of the associated chain or cable, such chain or cable will jerk the plow blade to a sudden stop, transmitting a sudden and sharp jolt of force back to the vehicle through the mounting structure. As will be appreciated, such bouncing of the plow blade happens repeatedly as the vehicle is driven or transported between locations.
As will also be appreciated, the suspension system on the front of the vehicle exacerbates the plow blade bouncing problem. Moreover, the effect of the significant mass/weight of the plow blade on the vehicle suspension system is significantly magnified when considering the repetitive bouncing movement of the plow blade as the vehicle is driven from location to location. This repeated bouncing of the plow blade can adversely impact the vehicle's suspension system by causing significant and rapid wear and tear thereof. Moreover, repeated bouncing of the plow blade can result in damage to a vehicle frame and/or the plow blade mounting structure. Furthermore, repeated bouncing of the plow blade causes extreme tensile stress loading of the chains or cables holding the plow blade in a suspended position. Of course, if such chains or cables should snap or break, the plow blade will crash thus enhancing the potential for accidents not only with the vehicle having the plow blade mounted thereon, but with other vehicles in the vicinity. Also, the potential bouncing of the plow blade can interfere with the steerability of the vehicle. All of these problems may be further magnified by the likelihood of adverse weather conditions normally including snow and/or ice laden streets and highways.
Furthermore, similar problems and difficulties may be encountered when the plow blade is lowered to its operating position. In this position, the plow blade is in contact with the road or off-road surface the vehicle is traveling on. To ensure proper contact between the plow and the surface to be plowed, the lift assembly is positioned such that the chains or cables do not support the full weight of the plow. The surfaces to be plowed, however, are commonly marred with uneven portions such as the joints associated with misaligned road surface segments, speed bumps, ruts, potholes and the like. When the plow blade contacts such imperfections in the road surface, the plow blade may be forced initially upward and then subsequently downward back to the road. Without any plow suspension system, the plow blade will freely plummet back to the ground or, if the surface imperfection is large enough, will snap to a sudden stop as the chains or cables are drawn taut. The resulting forces can be quite severe, and these forces are transmitted back to the vehicle through the mounting structure. As will be appreciated, such displacement of the plow blade may happen repeatedly as the vehicle operates to plow road and similar surfaces.
Thus, there is a need and a desire for a plow blade suspension system capable of attenuating shocks normally imparted to a vehicle by a plow blade as the vehicle moves over and across uneven terrain.
U.S. Pat. No. 4,947,563, issued to Paul T. Pfister, Jr., discloses a vehicle plow-suspension shock-absorber. Pfister involves a compression spring situated in line with the chain or cable that interconnects the plow blade to the lift arm. By adding such a compression spring, downward forces on the plow blade relative to the vehicle are dampened when the plow blade is in the raised storage or suspension position. Pfister, however, does not dampen the upward movement of the plow blade, and has little if any effect when the plow blade is in the lowered working or operational position.